docs: def surviors_are_inconsistent(survivor_mapping: Mapping[BaseEntity, Set[BaseEntity]]) -> Set[BaseEntity]:
      """Check that there's no transitive shit going on."""
      victim_mapping = set()
      for victim in itt.chain.from_iterable(survivor_mapping.values()):
          if victim in survivor_mapping:
              victim_mapping.add(victim)
      return victim_mapping